Reproduction is a fundamental feature of life. Animals usually require the differentiation of germ cells to create gametes. The gametogenesis process is delicately regulated, defects in which lead to fertility problems that impact 15% of humans. PIWI-interacting RNAs (piRNAs) are the most recently discovered and rapidly evolving class of small RNAs in animals. In vertebrates, piRNAs are almost exclusively expressed in testis, and they are essential for spermatogenesis. Some piRNAs guide PIWI proteins to silence transposable elements through sequence complementarity; humans, mice, and rooster, however, also produce a distinct and abundant set of non-repetitive piRNA sequences whose targets are unknown. My long-term goal is to understand the biology of piRNAs and their functions in male fertility. There is no obvious conservation of piRNA species, and piRNAs are heterogeneous: an organism carries millions of different piRNA sequences. The diversity is a result of their unusual biogenesis. Long single-stranded piRNA precursors are cleaved into thousands of snippets, and these fragments are loaded into PIWI proteins and become piRNAs. This fragmentation, which is poorly understood, distinguishes piRNA processing from other RNA fates. My previous work defined the precise transcription units of piRNA producing loci in mice, paving the way for the analysis of their post-transcriptional processing and their functions. I have further developed new methodologies and animal models to overcome longstanding roadblocks in piRNA research. In unpublished work, we found that ribosomes appear to participate in piRNA biogenesis, and that piRNAs act to regulate sperm epigenomes including RNAs and histones. Here, I propose to focus on two questions: 1) What principles designate a sequence to become a piRNA? 2) What is the function of piRNAs beyond transposable element silencing in male reproduction? The findings from this work may lead to the discovery of new mechanisms in male fertility and RNA biology.